It has heretofore been known that in certain instances high temperature reactions of silicate films with hydrogen can be utilized for certain purposes, such as the production of hydroxide-free silica for optical glasses. It is also known to those skilled in the art that products known as "hydrogen clays" can be produced by aqueous reaction of clays with mineral acids, as for example is described in U.S. Pat. No. 3,201,197. Such reactions have substantially no bearing upon the present invention, as will henceforth become evident.
In my U.S. Pat. No. 4,764,495, issued Aug. 16, 1988, there is disclosed a method for producing a layered lattice silicate which is surface-modified with an organic material, by pretreating the silicate such as a kaolin in a hydrogen atmosphere, and then reacting the product with a suitable organic compound. In my application U.S. Ser. No. 943,669 filed Dec. 17, 1986 there is disclosed a method for producing a layered lattice silicate which is surface-modified with an organic material, by contacting the silicate such as a kaolin with an organic monomer, co-monomers, or a prepolymer, and effecting surface polymerization or reaction in situ in the presence of a gaseous hydrogen atmosphere.
In accordance with the foregoing it is an object of the invention to provide a relatively simple and effective process for modifying the surface of a particulate aluminosilicate clay by forming on the surfaces thereof a silicon compound not previously present, from materials native to the clay.
In my application U.S. Ser. No. 114,538 filed Oct. 29, 1987 now U.S. Pat. No. 4,810,580, a clay such as a hydrous kaolin clay, i.e., a kaolin that has not been subjected to calcination at temperatures of 450.degree. C. or higher, can be coated with a silicon nitride layer when reacted at above 1000.degree. C., preferably 1100.degree. to 1500.degree. C., in a gaseous NH.sub.3 atmosphere. The novel product is gray in color, will not disperse in organic solvents and is more abrasive than a normal calcined kaolin (reacted in air). ESCA analysis of the silicon nitride coated product displays a split in the silicon peak with the new peak shifting from 105.8 to 106.4 ev. It is theorized that nitridation occurs in situ of the silicon atoms of the clay, which remain bound to the clay in the process, and that the nitridation forms structures comprising Si, N or Si, N, O or Si, Al, O, N.
An article by Sang Wook Choi et al in the Journal of the Korean Ceramic Society, vol. 23(5) 17-24 (1986), published on Oct. 30, 1986, describes the synthesis of products named B' sialon from compositions containing kaolin, graphite and metal silicon or aluminum at temperatures in the range of 1300.degree.-1450.degree. C. under a gas of 90% N.sub.2 -10% H.sub.2 for 20 hours. A composition of kaolin and graphite was also treated to give B'-sialon in major amount and .alpha.-Al.sub.2 O.sub.3 in minor amount. The graphite functions as a reducing agent and a source of carbon. The reactions are termed carbothermal reduction-nitridation.
An article by Y. Sugahara et al in Journal of American Ceramic Society, vol. 71, C-325 to C-327, July 1988, describes the production of silicon carbide and AlSiC materials from montmorillonite.
It is a further object of the invention to provide a relatively simple, inexpensive, and effective process, which enables surface modification of a clay mineral, such as a kaolin clay, by surface carbide groups, thereby providing a new product having valuable commercial applications.
Persons are incorporating ceramic fibers, whiskers, and particulates into ceramics to produce advanced composites, with improved properties such as fracture resistance, strength, and high-temperature stability. Ceramic matrix composites have been made by adding a variety of ceramic particulates, fibers and whiskers (very strong single crystals that are at least 10 times longer than they are thick) to a ceramic matrix to make the ceramic tougher by deflecting cracks. These fillers can dissipate the crack's energy through fractional forces. They also can brace the crack, keeping it from opening further.
Whiskers and particulates are more easily handled than continuous fibers. They can be handled like a fine powder, that being their appearance in bulk form. (They are about 0.5 um in diameter and whiskers are about 40 to 50 um long.) Because of their size, they can be easily mixed into the "batter" from which many ceramic components are fabricated.
The most commonly used whiskers are those made of silicon carbide. They are very strong and have been successfully incorporated into a variety of matrices, including alumina, mullite, zirconia, silicon nitride, and glass ceramics. Many studies have shown that the fracture toughness and/or fracture strength of polycrystalline ceramics can be improved by reinforcing them with single crystal silicon carbide whiskers, see C&EN, Feb. 1, 1988, page 7.
Silicon carbide does not exist in nature and must therefore be synthesized. This is done using pure quartz sand, coke containing a low proportion of ash and additions of sawdust and sodium chloride, the mixture being heated to 2500.degree. C. in an electric resistance furnace. About 6 kg raw material and 4.3 kWh electrical energy are required for 1 kg SiC. Owing to heat losses, however, energy consumption almost doubles--the figure 7.8 kWh/kg shows that this is really an energy-consuming and thus costly process, see N. N. Ault, and R. G. Robertson, "Silicon Carbide," Ceramic Bulletin, vol. 65 (1986) 741-742, and The Benecke: Hartstoffe in der Eisen-und Nichteisen Metallurgie, VdEh-Seminar 15/83, Monchengladbach Marz 1983; also Interceram, No. 1, 1988, pp. 35-39, an article by K. Hunold, H. Reh, Fed. Rep. Germany.